1. Field of the Invention
The present invention relates to a motor lock detection circuit.
2. Description of the Related Art
When a motor is locked to become incapable of rotating by a driving force from a motor driving circuit, an overcurrent flows through the motor driving circuit to cause heat generation. In this case, if the motor driving circuit is configured as an IC, the IC may be destroyed. For this reason, the motor driving circuit is provided with a motor lock detection circuit that detects lock of the motor with using a square-wave-shaped FG signal having a frequency corresponding to the rotation speed of the motor (see, e.g., Japanese Patent Application Laid-Open Publication No. 2006-129671).
FIG. 5 is a diagram of a configuration of a motor lock detection circuit 300 and a peripheral circuit thereof, and FIG. 6 is an explanatory view of an operation of the motor lock detecting circuit 300. A motor shown in FIG. 5 is a sensor-equipped single phase motor including a single phase drive coil L and a Hall element 2 for detecting a rotor position. Drive transistors T1 and T2 are connected to one terminal TA of the drive coil L, and drive transistors T3 and T4 are connected to the other terminal TB thereof.
An energization control circuit 30 energizes the drive coil L by appropriately turning on and off the drive transistors T1 to T4, to rotate the single phase motor. As a result, the Hall element 2 outputs a rotor position detection signal H+ with a sine-wave shape and a rotor position detection signal H− with a cosine-wave shape. A hysteresis comparator 4 generates a full-wave rectification waveform using the rotor position detection signals H+ and H− output from the Hall element 2, and slices the full-wave rectified waveform at a predetermined level to generate a square-wave-shaped FG signal shown in FIG. 6.
An edge detecting circuit 10 outputs an edge detection signal EDGE of H level shown in FIG. 6 to the base electrode of an NPN transistor B1 when detecting an edge of the FG signal output from the hysteresis comparator 4. The NPN transistor B1 is turned on when the edge detection signal EDGE is at H level, and is turned off when the edge detection signal EDGE is at Low level. The current amplification rate of the NPN transistor B1 is determined so as to create a current twice as large as a constant current I (2I) from a constant current source 40.
When the NPN transistor B1 is off, the constant current I from the constant current source 40 flows into a capacitance element C, so that the capacitance element C is charged corresponding to the constant current I. While the capacitance element C is charged, a connection point voltage Vc at the connection point between the collector terminal of the NPN transistor B1 and one terminal of the capacitance element C rises in voltage level. Then, when the NPN transistor B1 switches from OFF to ON, it becomes necessary to cause a current twice as great as the constant current I to flow through the collector-emitter passage of the NPN transistor B1. This causes the constant current I corresponding to charges accumulated on the capacitance element C to flow through the collector-emitter passage of the NPN transistor B1, so that the level of the connection point voltage Vc at the connection point between the collector terminal of the NPN transistor B1 and one terminal of the capacitance element C drops. This is, the level of the connection point voltage Vc rises when the NPN transistor B1 is OFF, and drops when the NPN transistor B1 is ON, as shown in FIG. 6.
However, when the motor is locked, the rotor position detection signals H+ and H− output from the Hall element 2 are fixed to a DC offset level, and therefore, the FG signal does not have a square waveform (see FIG. 6), so that the comparator 20 keeps outputting the edge detection signal EDGE of L level (see FIG. 6). As a result, the NPN transistor B1 continues to be OFF, and the capacitance element C continues to be charged with the constant current I from the constant current source 40, so that the connection point voltage Vc continues to rise (see FIG. 6). When the connection point voltage Vc exceeds a threshold voltage Vth, the comparator 20 outputs a detection signal DET of H level. At this time, the energization control circuit 30 detects lock of the motor based on the H-level detection signal DET from the comparator 20. The energization control circuit 30 then turns off the drive transistors T1 to T4 to stop energizing the drive coil L in carrying out motor protection control.
In a case of the motor lock detection circuit as shown in FIG. 5, the capacitance variation of the capacitance element C, which is charged and discharges for detecting motor lock, greatly affects a detection time or detection precision in detecting motor lock. For example, if the capacitance of the capacitance element C is excessively larger than a proper capacitance in relation to the threshold voltage Vth, a time of charging the capacitance element C becomes longer. Because of this, when the motor is locked, the connection point voltage Vc takes a long time to exceed the threshold voltage Vth, so that it needs a long time to detect lock of the motor since the motor is locked in actuality. On the other hand, if the capacitance of the capacitance element C is excessively smaller than the proper capacitance, the connection point voltage Vc takes a short time to exceed the threshold voltage Vth, which causes misdetection due to noises generated in the connection point voltage Vc that the motor is in a locked state despite of the fact that the motor is not locked in actuality.